Communication equipment represented by mobile phones is required to integrate communication functions that can handle various communication systems and various frequencies into the same equipment. Further improvements of the functionality and miniaturization are also demanded strongly. On one semiconductor chip, for example, a circuit is constructed such that an active element block represented by a transistor to treat signals such as digital signals and analog signals at radio-frequency is combined with a passive element represented by a resistor, a capacitor, or an inductor. In a radio-frequency integrated circuit, particularly, various levels of signals are treated including very weak signals used for reception (e.g., a signal level of about −100 dBm) and large signals for transmission (e.g., a signal level of about +10 dBm). In order to operate the integrated circuit as the circuit design, the wave form of the fundamental signals have to be less distorted, and signals treated on one circuit have to be prevented from crosstalk to another neighboring circuit or interfering with signals of the neighboring circuit mutually on the semiconductor substrate of the radio-frequency integrated circuit.
The passive elements used for radio-frequency integrated circuits, which are represented by a resistor, a capacitor, or an inductor, cannot be operated at radio-frequency unless the resistive loss component and the stray capacitance component are small, and the constructed circuit has a high Q-factor. Moreover, the loss increases to increase the power consumption, making it difficult to operate portable devices such as mobile phones for a long time with batteries. Accordingly, the resistive loss component and the stray capacitance component of the passive element have to be extremely small.
As these radio-frequency integrated circuits, so-called bonded semiconductor wafers have been put to practical use and largely used recently; in which a bonded semiconductor wafer, in other words, a so-called trap-rich type SOI (Silicon on Insulator) substrate, has a base wafer composed of a silicon single crystal, a polycrystalline silicon layer (also referred to as a trap-rich layer) on the base wafer, a dielectric layer on the polycrystalline silicon layer, and a single-crystal silicon layer on the dielectric layer. In the base wafer used for this case, distortion of the radio-frequency fundamental wave and crosstalk signals decrease as the specific resistance is higher. Accordingly, wafers of about 1 kΩ·cm to 4 kΩ·cm have been used commonly in view of mass productivity of base wafers. Herein, the decreased distortion of radio-frequency and crosstalk signal can be determined by measuring secondary harmonic wave characteristics (the ratio of component that has twice frequency of the fundamental frequency), meaning small secondary harmonic wave.
The polycrystalline silicon layer is deposited to prevent inversion of the base wafer lain thereunder. The polycrystalline silicon layer with the thickness of about 1 μm to 2 μm has been used in view of balance of physical warpage and distortion of the whole SOI substrate.
Regarding the specific resistance value of a base wafer and mass productivity thereof, as the specific resistance is lower, the impurities can be controlled easier, which enables mass production of substrates with the targeted specific resistance. In the present mass production technology of silicon single crystals, however, high specific resistance, for example, more than 4 kΩ·cm is difficult to realize since the targeting involves controlling to decrease the impurities. Under the present conditions, it becomes impossible in an extreme case to predict whether the wafer shows a value near 4 kΩ·cm or a value near 8 kΩ·cm until the wafer is actually prepared. That is, in an industrial view, the wafers are produced under extremely unstable conditions. As a result, base wafers with a high specific resistance have been produced in poor yields with very high price. This causes an increase of price of semiconductor chips for mobile phones and smart phones, which are main market of the radio-frequency integrated circuit, losing any industrial value.
If the mass production of base wafer with a high specific resistance can be realized, other large problems remain.
The first problem is a shortcoming that the specific resistance is liable to change by heat treatment due to an influence of donors formed from oxygen contained in the base wafer itself, which is caused by extremely low impurity concentrations as follows: an impurity concentration of phosphorus is about 3×1012/cm2 in an n-type semiconductor with the specific resistance of 1 kΩ·cm, and an impurity concentration of boron is about 1×1013/cm2 in a p-type semiconductor with the specific resistance of 1 kΩ·cm. It becomes possible to prevent this fluctuation of specific resistance to a certain extent by setting the oxygen concentration of a base wafer to low and adjusting the heat treatment temperature used for the semiconductor process.
The second problem is formation of a layer with a lower specific resistance by forming an inversion layer on the front face side of a base wafer with a high specific resistance due to electric charge contained in a so-called BOX oxide film (a buried oxide film) or electric charge captured on an interface level that appears on the interface between the BOX oxide film and the polycrystalline silicon layer. Such formation of the layer with a lower specific resistance promotes crosstalk of radio-frequency signals to abandon the meaning to use a base wafer with a high specific resistance. In a so-called trap-rich type SOI substrate, a polycrystalline silicon layer (a trap-rich layer) is inserted to prevent such formation of an inversion layer. However, the oxide film remains when applying insufficient temperature conditions for depositing the polycrystalline silicon layer or insufficient hydrogen treatment for removing the surface oxide film before the deposition. This causes formation of an inversion layer again, regardless of introducing the polycrystalline silicon layer, under the oxide film lying thereunder to abandon the meaning to use a base wafer with a high specific resistance.
The third problem is impurities such as phosphorus and boron that can be involved immediately under the dielectric layer, which is referred to as a so-called BOX oxide film, in a bonding step or a step of oxidation or heat treatment using an electric furnace when manufacturing a trap-rich type SOI substrate. The impurities are dispersed into the polycrystalline silicon layer and the base wafer to cause large lowering of the specific resistance of the polycrystalline silicon layer and the base wafer. Regarding this diffusion of impurities, it has been considered that the diffusion source is mainly originated from impurities contained in the air of a so-called clean room and pure water used for a semiconductor process, as well as impurities of other products that have remained in oxidation or heat treatment using the electric furnace. The measurement of the impurity concentration itself is technically difficult since the impurity concentrations are extremely low such that an impurity concentration of phosphorus is about 3×1012/cm2 in an n-type semiconductor with the specific resistance of 1 kΩ·cm, and an impurity concentration of boron is about 1×1013/cm2 in a p-type semiconductor with the specific resistance of 1 kΩ·cm. As a semiconductor substrate having a high specific resistance for radio-frequency, the base wafer are required to have a specific resistance such as 5 kΩ·cm and 10 kΩ·cm. It is almost impossible to maintain a clean room to treat such substrates and an electric furnace such that contamination of the clean room as well as re-addition of impurities in the electric furnace and diffusion of the impurities are reduced to very low levels.
In production of normal semiconductors that are not for radio-frequency, impurities contained in air of a so-called clean room and pure water used for semiconductor processing have not been considered as a problem. This is only because the semiconductors may be wafers that are required to have a specific resistance of 100 Ω·cm to 1 kΩ·cm or less, corresponding to a high impurity concentration level of about 1×1014/cm2, which enables normal environmental control methods for clean rooms to handle this impurity concentration of about 1×1014/cm2. In such clean rooms for semiconductor process to produce normal semiconductors, which are not for radio-frequency, the concentration of unexpected impurities is high. Accordingly, it has been impossible to test-manufacture or produce, for example, a trap-rich type SOI substrate for radio-frequency and an integrated circuit operated at radio-frequency using the same, each of which is required to have a higher specific resistance.